the reason it will not give a recipy is because the wanted pH is too far removed from the buffer's pKa value. And the further the actual pH is removed from it, the weaker the buffer strength will become.
In short, you can make a solution with pH 6 with the phosphate couple, but it will not be a good buffer to keep the pH at 6, small additions of acid can shift the pH negative decently fast at that point.
you said you wanted just potassium ions and (hydro-)phosphate ions in solution, hence my suggestion in using KOH and phosphoric acid to complete the buffers if you have a pH electrode to see the pH. HCl would add chlorides to your solution. If chlorides arent a problem, by all means use hydrochloric acid.
remember, if you do not have a way to read the pH of the solution (using that pH electrode) the best way is probably by calculating the desired ratio of monohydrophosphate and dihydrophophate.
Hope this helped you.